1. Field of the Invention
The present invention relates to an optical module provided with an optical fiber block and a microlens substrate or an optical module provided with the optical fiber block and a diffraction grating, and a method of forming such an optical module.
2. Description of the Prior Art
An optical module combining an optical fiber block with a microlens substrate is used as a device for optical communication. This optical module is designed to cause light from a light emitting diode to enter the microlens through an optical fiber to take it out as a collimated beam and to cause the collimated beam to enter the optical fiber through the microlens. Such a concrete structure is shown in FIGS. 14(a)˜(c).
The optical module shown in FIG. 14(a) is provided in which an optical fiber block 102 into which an optical fiber 101 is inserted is integrally formed with a microlens substrate 104 on which a microlens is mounted, through a transparent spacer 105. On the other hand, the optical module shown in FIG. 14(b) is provided in which the optical fiber block 102 is directly secured to the microlens substrate 104. The optical module shown in FIG. 14(c) is provided to have the optical fiber block 102 and the microlens substrate 104 secured to a base 106.
Referring to FIG. 15, a plurality of optical fibers 101 is inserted into the optical fiber block 102 and a plurality of microlenses 103 corresponding to the plurality of optical fibers 101 is mounted on the microlens substrate 104. This structure is for example disclosed in Japanese Unexamined Patent Publication No. HEI 2-123301 (1990).
The microlens substrate 104 is made for example by a method whereby an area with a different refractive index is formed on a surface of a glass substrate by ion exchange through a mask, a method whereby resin of high refractive index is embedded in a depression formed by etching, a photopolymer molding method whereby ultraviolet-curing resin is press-molded on a surface of the glass substrate, or a sol-gel method.
On the other hand, the optical fiber block 102 is provided in which V-grooves are formed at regular intervals on a silicon substrate or the like and optical fibers are fixedly secured within these grooves. In addition to this structure, it is also known that a stainless steel substrate or a glass substrate is provided with holes in advance into which the optical fibers are inserted and fixedly secured.
In the optical module, it is essential for the optical fiber to coincide with an optical axis of the microlens for improvement of communication accuracy. If their optical axes do not coincide, an object beam does not coincide with the optical axis as shown in FIGS. 14 and 15.
A method of adjusting such an optical axis of a module is proposed and disclosed in Japanese Unexamined Patent Publication No. HEI 9-061666 (1997). In this prior art, a mask with a mesh-shaped pattern of the same array pitch as that of an optical fiber block and a collimating lens array (i.e., a microlens substrate) is disposed in front of a detector for detecting the shape of a light beam to cause the light to enter the collimating lens array through the optical fiber block. The light transmitted through the collimating lens array without being shielded by the mask is detected by the detector to adjust a relative position of the optical fiber block and the collimating lens array so that the light beam shape corresponding to each optical fiber can be equalized.
In the conventional methods of adjusting the optical axis including the method disclosed in Japanese Unexamined Patent Publication No. HEI 9-061666 (1997), since the adjustment is made by moving either of the optical fiber block or the microlens substrate, a special moving device is required and the adjustment itself becomes very rough.
In particular, many optical fibers are united with many microlenses in some optical modules. The microlense substrate can be provided with many microlenses in one or two-dimensional way with a comparatively high accuracy. However, in the optical fiber block, making the grooves or holes is an essential process. Further, an external diameter of the optical fiber itself is uneven and a core position for the external diameter of the optical fiber is also uneven. Accordingly, many optical fibers cannot be arranged in one or two-dimensional way with a high accuracy.
Further, in the optical fiber block consisting of many optical fibers, the direction and amount of deviation or misregistration of each optical fiber are uneven as shown in FIG. 13. Accordingly, even though the optical axis is adjusted in accordance with one optical fiber, the optical axis adjustment of the other optical fibers may be deteriorated.